A high-density recording is achieved in a recording/reproducing apparatus such as a hard disk drive, a CD drive, a DVD drive, and a BD drive. The recording/reproducing apparatus employs a Partial Response Maximum Likelihood (PRML) signal processing scheme.
In a conventional recording/reproducing apparatus, a binarizing scheme for a reproduced signal, a waveform slice scheme is employed. In the waveform slice scheme, binarization is performed depending on whether amplitude of a reproduced signal is higher or lower than a threshold level. However, since the amplitude of the reproduced signal decreases due to high-density recording, a large number of identification errors occur in the binarization performed by the waveform slice scheme. It is known that the PRML signal processing scheme can obtain a higher signal quality than that in a conventional level slice scheme, even in information recorded with a high density.
In the PRML signal processing scheme, through a PR equalizer, an input signal is supplied to a Viterbi equalizer, and a decoding signal is output from the Viterbi equalizer. The Viterbi equalizer cumulatively sums errors between actual input signals at sample points and all supposed paths and selects a path having a minimum cumulative sum. A bit string corresponding to the selected path is output as a decoded signal. In the PRML signal processing scheme, the accuracy of decoding is determined according to what kind of partial response the PR equalizer equalizes the input signal to.
In the past, an apparatus has been developed, in which a reproduced signal is supplied to a plurality of PR equalizers, which equalize the reproduced signal to a plurality of partial response signals. Then, the plurality of obtained partial response signals are weighted with an appropriate ratio and are added up, and the obtained partial response signal is input to a Viterbi equalizer. However, this apparatus includes a small effect for improving a characteristic (bit-error rate). This apparatus simply derives one metric from the plurality of PR targets and performs Viterbi equalization. Therefore, even an apparatus for equalizing one PR target can obtain substantially the same degree of characteristic (bit-error rate) as this apparatus when a tap coefficient of an FIR filter constituting a PR equalizer is selected appropriately.
It should be noted that a reception apparatus for decoding a transmission signal may sometimes employ PRML signal processing scheme, and in such case, the same phenomenon occurs.